Buffer notifications option

I added a new option to pass to node-apn.

Documented as: @config {Boolean} [buffersNotifications=true] Whether to buffer notifications and resend them on reconnections.

If buffersNotification is true, node-apn will buffer notifications as done until now, and resend them as soon as reconnecting to apple's servers. If false, the notifications won't be buffered and resent.

This can be useful, as is my case, if developing an http push server, which will answer to the clients whether the push has failed or not. If the push fails to be delivered to apple, node-apn shouldn't try to resend it as the client application doesn't expects it to be sent.

Josep